Metal oxide semiconductor field effect transistors (MOSFET) can be constructed to conduct large currents, on the order of several amperes. These high current MOSFETs may be employed within computer disk drives, power supplies, telecommunication devices, consumer electronics and many other devices. Within computer disk drives, high current MOSFETs may be employed as power switches in the drive electronics for the spindle motors, as shown in FIG. 1.
A schematic diagram of one phase of the drive electronics for a three-phase disk drive spindle motor 100 is shown in FIG. 1. Spindle motor 100, simplified for illustration, includes three stator coils, 102, 104 and 106. During operation of motor 100 the stator coils are selectively energized to cause the motor shaft or rotor (not shown) to turn at a desired rotational speed. Energizing a stator coil causes it to behave as a magnet with its strength and polarity determined by the amount and direction of current flow in the coil.
Energization of the stator coil 102 is controlled by commutation logic 110 and power transistors M.sub.1 and M.sub.2. Transistor M.sub.1 is combined with an amplifier A.sub.1 and sense resistor R.sub.1 to form a negative feedback loop that controls the current supplied to stator coil 102. Commutation and control circuitry 110 controls the switching of power transistor M.sub.2 and provides an input, identified as V.sub.IN, to amplifier A.sub.1 to control the operation of transistor M.sub.1. The voltage signal V.sub.1 determines the current in the power transistor M.sub.1 and M.sub.2 which in turn forces motor 100 to turn at a desired speed.
The construction and operation of the commutation logic, drive electronics and spindle motor as shown in FIG. 1 and discussed above are well known by those skilled in the art.
In past disk drive designs the power transistors that are used to drive the spindle motor have been discrete devices not integrated with any other circuitry. The fabrication of high current or high voltage power electronics on the same chip as with logic or other circuitry has required unique process considerations or chip architectures. In recent years, however, as the form factor sizes and power requirements of disk drives have decreased, some integration of power transistors with other circuitry has been accomplished in typical power semiconductor technologies such as bipolar and DMOS. Such integration is limited though, as many restrictions remain which constrain the integration of power circuitry together with other circuitry. The operating parameters and output requirements of the power electronics restrict such integration to specific applications.
The reduction of disk drive form factor size and power requirements has reduced the current requirement for a typical disk drive spindle motor to approximately 900 mA. It is probable that this value will be further reduced in the future as disk drive form factors and power requirements continue to shrink. However, the metalization and bond wire utilized in CMOS technology is not of adequate thickness to support current levels near 900 mA.